The present invention relates generally to the manufacture of plastic bags from laminated sheet material including plies of thermoplastic material and, more particularly, to a process and apparatus for sealing plies of thermoplastic material included in laminated sheets having thickened sections, such as zipper closures, during the manufacturing of such plastic bags.
The manufacture of plastic bags such as trash bags, folded sandwich bags and other nonzippered bags from uniform film stock has been perfected for many years. Such plastic bags have two generally parallel walls which are joined together at their respective mated sides and bottom edges to form a container for items to be held within the bag. The upper edges of the two walls adjacent one another are open to provide access to the bag. Quality seals are produced during the manufacture of these bags at high cycle rates by a variety of sealers of cutter/sealer devices due to the uniformity of thickness of the film stock used to form the bags.
As an improvement to such plastic bags, any one of a variety of openable and recloseable interlocking "zipper" closures may be fitted or integrally formed into the open ends of the bags. The provision of closure devices leads to difficulty in manufacturing the bags since the multiple plies of thermoplastic material then include portions of nonuniform thicknesses due to the closures which have substantially greater thickness than the remainder of the film. The thicker zippered closure portions of the bags require a sealing device to be operated at higher temperatures and lower cycle speeds in order to adequately seal those portions of the bags.
To add to the sealing difficulties for closeable plastic bags, flexible laminated sheet material may be used for their construction. For example, the bag may have walls of a laminated sheet material in order to protect electrostatically sensitive components from potentially damaging electrostatic charges. However, difficulties may arise from the potential for separation of the plies of the laminated sheet material due to the manufacturing process. But the integrity of such bags is important because it assures dissipation and/or shielding to protect against electrostatic charges. In addition, bags of this type have also been utilized in the medical and pharmaceutical industries for containing ultra clean instruments and drug substances. Again the integrity of such bags is important because the medical contents might otherwise be contaminated by dust and other airborne contaminants in the event of a build up of electrostatic charges on packaging materials.
During manufacturing operations, it was discovered that in plastic bags formed from laminated sheet materials, such as that utilized for electrostatic dissipative and/or shielding packages, weak side seals were developing, particularly around the zipper closure portions of the bags. To overcome these problems, higher temperatures and pressures were used in existing sealing equipment. While the use of higher temperatures and pressures added to the strength of side seals in the bags formed from laminated sheet material, it was discovered that the plies adjacent the zipper closure portions of the bags tended to delaminate from the other plies. Thus, when applying known techniques to improve seal strength, delamination problems arose which counteracted the improvements in seal strength.
Consequently, a need exists for improved sealing of the side seals of bags made from laminated sheet material having nonuniform thickness film stock, primarily film having thickened sections due to the presence of zipper closure portions, which sealing not only provides strong side seals to retain items contained within the bags but also forms the seals such that the plies of the laminated sheet material from which the bags are manufactured do not delaminate from one another, particularly around the zipper closure portions of the bags.